With current treatment options, the five-year survival rate for patients with AML is only 25%, thus understanding mechanisms of relapse and resistance is important therapeutically. The most common aberration in AML results in activation of the fms-like receptor 3 (Flt3-ITD), which was recently shown to alter the redox environment of cancer cells. While these changes remain poorly understood, elevations of reactive oxygen species can promote the cancer phenotype. Heme oxygenase 1 (HO-1) is a ROS-induced anti-oxidant that can also inhibit apoptosis and promote metabolism leading to cell survival and drug resistance. Our preliminary data suggest that, Flt3-ITD-expressing cells contain elevated constitutive expression of HO-1 as compared to Flt3-wildtype (WT) expressing cells. Further, knockdown of HO-1 with siRNA results in decreased proliferation and viability of Flt3-ITD expressing cells. We hypothesize that Flt3-ITD-dependent kinase signaling and ROS production increase constitutive expression of HO-1, leading to activation of anti- oxidant and anti-apoptotic pathways and resulting in proliferation and drug resistance in AML. To test our hypothesis, we will 1) Determine the molecular mechanisms by which HO-1 is constitutively expressed in Flt3- ITD positive AML and the functional consequences of this expression and 2) Elucidate the role of HO-1 in therapeutic resistance of AML. The isogenic BaF3 Flt3-WT and Flt3-ITD cell lines will be used in combination with human Flt3-ITD positive AML cell lines MOLM-13 and MV4.11 to test both specific aims. To complete Aim 1, we will examine the Flt3-kinase and ROS dependence of HO-1 expression in Flt3-ITD positive cells. Transcriptional regulation of HO-1 expression will also be evaluated. In vitro findings will be further assessed ex vivo utilizing tisue microarray specimens from AML patients. The effects of HO-1 expression on Flt3-ITD positive AML will be delineated utilizing chemical and genetic inhibitors of HO-1. For Aim 2, we will determine the contribution of HO-1 to chemoresistance of Flt3-ITD expressing cells utilizing the afore mentioned HO-1 inhibition methods. Using similar techniques, we will also evaluate the contribution of HO-1 to de novo and acquired resistance to the Flt3 inhibitors lestaurtinib and sorafenib. These studies together will provide a greater understanding of the redox mechanisms underlying Flt3-ITD expressing AML, and will better equip us to therapeutically target the biological pathways that promote AML progression and drug resistance.